This disclosure relates to a cutting tool and a method of machining.
A rotor of a typical gas turbine can have more than one hundred blades that are secured to rotating disks. The blade/disk attachment scheme includes providing a root on each blade that is secured to the disk. One type of root used to attach blades, commonly referred to as a “fir tree,” includes a convoluted section that has a complementarily shaped slot in the disk periphery.
Broaching is one common method used to manufacture fir tree slots. Broaching nickel-based super-alloys may induce undesired surface characteristics such as excessive material strain hardening and surface microstructure alteration, which typically results in an uneven surface due to deflection during machining. A white-etched layer (WEL) on the surface is one indication of undesired surface characteristics. Aside from the high cost of the broach tools and limited tool life, part scrap rate will increase due to the defected surface integrity.
Several mechanical-based approaches are used to remove some of these undesired surface characteristics that remain after broaching. The efficiency of these approaches depends on the thickness of the WEL and depth of surface microstructure alteration. For example, the broached disk typically is moved to a second machining setup to perform post-processing operations to remove the undesired surface characteristics. Some known post-processing operations include performing thermal treatment on the broached part by heating and rapid cooling, induction heating, and more recently, laser heat-treating. Such thermal treatment has typically been employed to increase surface hardness subsequent to broaching.
Although heating and rapid cooling increases surface hardness, this process also induces an internally stressed microstructure that makes the surface brittle. To relieve these internal stresses, a subsequent tempering process is required that typically entails heating the part to a temperature below the critical temperature for several hours. Although tempering facilitates avoidance of a microstructure phase change, the surface hardness is also undesirably reduced.
In induction heating, the part surface is placed within an induction coil. As electrical current in the induction coil is increased, the surface is heated above the critical temperature, thus also causing a microstructure phase change. When the surface is rapidly cooled, or quenched, a new microstructure phase is formed. Only a shallow depth beneath the surface is heated above the critical temperature and is rapidly quenched, while the remainder of the part remains unchanged. However, the rapid cooling also induces internal stresses that cause the surface to become brittle. Although a subsequent tempering process is required to relieve the internal stresses, tempering is time consuming. Additionally, the heat treated depth is controlled during induction hardening by producing a higher frequency current in the induction coil. However, common induction heating machines and surface area present limitations based on the highest frequency available.
Hardened steel surfaces have been produced by machining using laser processing. For example, a method of laser heat-treating a flat part, such as a knife or blade, has been employed by focusing a laser beam perpendicular to the major flat surface of the part using a cylindrical lens. The width of the beam is adjusted according to the desired width of the part to be heated. The part is then moved through the laser or the laser may be moved along the part to heat the surface. There is no subsequent machining of the surface of presence of a WEL.
It is desirable to provide a method of eliminating undesired surface characteristics during broaching without inducing further defects in the part.